Significant Contribution of Ice Age Cycles to Early Human Interbreeding

An article published in the journal Science reveals that changes in climate throughout the last 400,000 years have impacted the interbreeding between Neanderthals and Denisovans.

In the intricate tapestry of human evolution, interbreeding has been revealed as a common thread among our ancestral lineages. Yet, a shroud of mystery enveloped the intricacies of when, where, and to what extent this crossbreeding occurred among early human species. A pioneering study¹, harnessing a synergy of paleoanthropological insights, genetic revelations, and supercomputer simulations of ancient climates, has started to illuminate these shadowy corners. Unveiling a remarkable nexus between inter-glacial climates and vegetation dynamics, this research discloses the once-concealed habitats that fostered interactions and gene flow between Neanderthals and Denisovans. This journey takes us through epochs of environmental change and offers profound insights into the intertwined destinies of these ancient human relatives.

Within the genetic archive of modern humans reside vestiges of Neanderthal and Denisovan lineage, a testament to the intricate mingling of our ancestral past. Emblematic of this genetic intermingling is "Denny," an individual whose ancient remains, dating back 90,000 years, echo the crossroads of Neanderthal and Denisovan heritage. However, the enigma persisted: when did the paths of these early human species intersect, and what landscapes bore witness to their convergence?

Published on August 10, 2023, in the esteemed journal Science, a collaborative endeavor between researchers from South Korea and Italy embarks on a journey to demystify these questions. Combining the mosaic of fossilized relics, climate simulations, and genomic revelations, this study seeks to chart the course of Neanderthal-Denisovan interbreeding. Dr. Jiaoyang Ruan, a Postdoctoral Researcher at the IBS Center for Climate Physics (ICCP) in South Korea, elucidates,

"In the realm of historical interbreeding between Neanderthals and Denisovans, gaps persist in our understanding of when, where, and with what frequency this occurred. By harmonizing species distribution models, which encompass a wealth of fossil, archaeological, and genetic data, with intricate simulations of global climate and biome shifts, we endeavored to illuminate the potential avenues for this genetic exchange."

The crux of their revelation lies in the intricate dance between these hominin species and the shifting backdrop of their environment. Initially, Neanderthals and Denisovans each gravitated towards distinct ecological niches, reflecting their adaptability to diverse climes. While Denisovans thrived in the frigid embrace of boreal forests and tundras, their Neanderthal counterparts found solace in the embrace of temperate forests and grasslands. Yet, the cosmos had a narrative shift in store: shifts in Earth's orbital dynamics set into motion a sequence of climatic fluctuations, triggering corresponding transformations in vegetation patterns. These shifts acted as a siren call, luring both species to overlapping habitats, a serendipitous embrace that elevated the prospects of interbreeding.

Through meticulous analysis, these researchers deciphered the nexus points of contact, where Neanderthals and Denisovans were destined to meet. Central Eurasia, the majestic Caucasus, the lofty Tianshan, and the venerable Changbai mountains emerged as pivotal hotspots of interaction. It is here that the intriguing tale of "Denny" unfurled, her lineage intricately woven into the climatic context, corroborating known instances of genetic mingling. Further exploration uncovered the Siberian Altai as a crucible of connection during critical epochs, spanning ~340-290, ~240-190, and ~130-80 thousand years ago.

Delving deeper, the study unveiled the mechanisms driving an "east-west interbreeding seesaw," a dance choreographed by the fluctuations in Eurasian vegetation over the past 400 thousand years. The symphony of elevated atmospheric CO2 levels and benign interglacial climates orchestrated an eastward expansion of temperate forests into central Eurasia, beckoning Neanderthals into Denisovan terrain. Conversely, the refrain of lower CO2 concentrations and ensuing glacial severity orchestrated a partition of their habitats, dampening the cadence of their interactions and the occurrences of interbreeding.

Elucidating this symphony's intricacies, Professor Axel Timmermann, Director of ICCP and a luminary at Pusan National University, South Korea, remarks,

"The pronounced shifts in climatic zones, emblematic of central Eurasia's overlap region for Denisovans and Neanderthals, stand as a testament to climate's sway over vegetation dynamics. These shifts, engendered by past fluctuations in atmospheric CO2 levels and northern hemisphere ice-sheet volumes, cast a profound influence on the ebb and flow of potential interbreeding events."

In the grand tapestry of human history, this study unearths a pivotal truth: climate, an unseen conductor, played a symphonic role in orchestrating the interactions between early human species. As our gaze spans epochs, the revelations from this research impart echoes that resonate through time, underscoring the indelible influence of environmental shifts on the genetic mosaic of modern humanity.

1. Jiaoyang Ruan, Axel Timmermann, Pasquale Raia, Kyung-Sook Yun, Elke Zeller, Alessandro Mondanaro, Mirko Di Febbraro, Danielle Lemmon, Silvia Castiglione, Marina Melchionna. Climate shifts orchestrated hominin interbreeding events across Eurasia. Science, 2023; 381 (6658): 699 DOI: 10.1126/science.add4459